关系代词指代整个主句内容，只能是非限制性关系分句，而that不能引导非限制性关系分句。换句话说，关系代词that不可能指代整个主句内容。所以你的理解是错误的。

其实，夸克大全 18. 42 讨论的是分隔修饰可能带来的歧义，以及如何避免歧义。如果你仔细阅读这段内容，你就应该明白这句话的结构（即that从句修饰protest，分隔后可能造成的歧义）

原文如下：

Other discontinuities
18.42 Some adjectives that take complementation (cf7.22, 16.68ff) can simultaneously
function as premodifiers. Compare:
This result is different from yours.

This result is similar to hers.

This is a different result from yours
This is a  similar result to hers.
In cases like these last, discontinuity is felt to be quite normal and the
position is similar to that of the correlative item in comparative sentences
(cf 18.41). So also:
She works in the opposite room to this
It is a timid dog with strangers.
Despite superficial similarity with prepositional phrases postmodifying a
head, there is rarely danger of ambiguity:
I was told by a man in the office.
The possible difference here ('A man told me in the office' or 'A man in the
office told me') is unlikely to give rise to communicational difficulties, but the
following example is more seriously ambiguous:
They made an embarrassing protest to the authorities.
In such instances, revision is essential to make it clear which of the two
possible meanings is intended:
They made a protest that was embarrassing to the authorities. [l]
[ie 'Their protest embarrassed the authorities']
They made a protest to the authorities that was embarrassing. [2]
[ie 'They protested to the authorities in a way that was
(generally) embarrassing']
If the meaning of [2] is intended but with the end focus on the authorities as
in [I], further revision is necessary. Perhaps:
They embarrassed everyone by making a protest to the authorities. [2a]